This invention relates to novel epoxy-styrene-polyester solventless resin impregnating compositions having excellent atmospheric catalyzed pot life, and the process for making such compositions. Foster, in U.S. Pat. Nos. 3,557,246 and 3,657,196, pre-reacted an epoxy resin and maleic anhydride to form a complex half-ester at about 85.degree. C. to 90.degree. C. To the half ester; styrene, Nadic methyl anhydride, benzoyl peroxide, and hydroquinone reaction inhibitor were added. This provided a resinous impregnating composition having excellent electrical properties and a storage lifetime of up to about 2 months. The composition was produced by a complex reaction mechanism however, which made it difficult at times to maintain consistent batch uniformity.
Foster et al., in U.S. Pat. No. 3,919,348, solved problems of batch uniformity by conducting an essentially complete esterification reaction, at about 80.degree. C. to 100.degree. C., between a liquid-solid epoxy resin mixture and maleic anhydride in the presence of a catalyst such as piperidine, pyridine, imidazole or tertiary amine; and then adding other ingredients such as styrene, and a nitrophenol stabilizer. As a final step a polycarboxylic anhydride and a peroxide or azo compound could be added.
Both of these compositions require hot blending of ingredients, to react the epoxy and anhydride, in order to produce a one-half or full esterification reaction. Such compositions require heating equipment for their manufacture. These systems do not allow small producers to buy the ingredients, and simply cold blend them in the quantities they need. What is needed, is a resin system that can be cold blended, to provide a solventless insulating composition having the properties of low viscosity at room temperature, ease of curing, good electrical resistivity, and long shelf life.